Polymer characterization

Solution viscosity

Viscosity number and intrinsic viscosity

Solution viscosity is a measure of the average molecular weight of a polymer. The determination is carried out on dissolved polymer, using different solvents (formic acid, m-cresol, tetrachloroethane, ...) and concentrations. The measurement in the capillary viscometer provides the viscosity number VN (ml/g).

The viscosity number makes it possible to control the processing and use properties of plastics. Thermal stress on the polymer, ageing processes or the effects of chemicals, weathering and light can be investigated by comparative measurements.

The method is standardized for common plastics, for example in DIN EN ISO 307 for polyamides and DIN ISO 1628-5 for polyester.

Other key figures are relative viscosity and specific viscosity, which describe the change in viscosity of the pure solvent caused by the polymer.

The intrinsic viscosity (also called limiting viscosity or Staudinger index) is the extrapolation of the viscosity number VN for the case of disappearing polymer concentrations. It can be estimated from series of measurements or by using suitable approximation methods (e. g. Billmeyer).

Melt flow index

Volume flow rate and mass flow rate

The melt flow index is often referred to by the abbreviations MFI (melt flow index) or MI (melt index). It is used to characterize the flow behaviour of a thermoplastic and thus its degree of polymerisation. Through comparative measurements, the MFI is suitable for revealing material impurities and processing defects. Therefore, the melt flow index is used as standard in quality assurance or failure analysis.

A distinction is made between the melt volume-flow rate (MVR) and the melt mass-flow rate (MFR). Both are linked via the melt density, the measurement method is described in DIN EN ISO 1133 and is a routine procedure in plastics analysis.

Soxhlet extraction

Extraction by organic solvents

DIN EN ISO 6427 describes a large number of possible methods for a wide variety of plastics and solvents. The method to be used depends on the material and the problem. The extracts allow statements to be made regarding the dissolved mono- and oligomers, plasticisers, non-cross-linked resin components, emulsifiers, ...

Moisture content of polymer granulates

The residual moisture is an important parameter in the further processing of plastics. Excessive moisture leads to injection moulding defects or even polymer degradation during further processing. With the help of the Karl-Fischer method described in DIN EN ISO 15512, the water content is specifically determined quantitatively by titration. Other emissions during heating of the pellets are not included in the measurement. The carbon sample is heated in an airtight sealed vessel and the released moisture is transferred to the titration unit via a carrier gas flow.

Ash content / ash residue

The ignition residue describes the residual mass of an organic substance after combustion and continuous heating at high temperatures until mass constancy is reached. It is a measure of the content of inorganic components in the polymer, such as glass fibres.

DIN EN ISO 3451-1 describes several methods for determining this residual mass, which is called ash or sulphated ash (depending on the method).